The present invention relates to methods and systems for monitoring combustion condition which are suitable for monitoring the combustion condition in a combustion furnace and a gas turbine in an incinerator, a thermal power plant or the like, and in an internal combustion engine.
In recent years, air pollution due to combustion off-gas discharged from large scaled combustion furnaces installed in incinerators facilities, heat power plants and so on and from internal combustion engines of automotive vehicles has become a grave social problem. Particularly, injurious materials such as dioxin has been discharged from incinerators for incinerating garbage in cities, industrial wastes and so on, and become a serious social problem.
To solve this problem, a number of countermeasures have been taken to the garbage incineration for improving incinerators themselves, improving combustion process control methods to accomplish optimal combustion, improving and developing technologies for eliminating hazardous materials, and so on, such that the production of such injurious materials is reduced. More specifically, these countermeasures are intended to accomplish complete combustion to minimize the production of injurious materials such as dioxin.
The dioxin group is a type of hydrocarbon containing oxygen substituted with chlorine, and is thought to be produced during a combustion process from unburned hydrocarbons, hydrogen chloride and so on. It is therefore contemplated that a reduction in unburned hydrocarbons can result in a reduction in production of dioxin group. For achieving the complete combustion, it is necessary to keep track of what serves as a measure of combustion and precisely control the combustion based on the measure. Any improvements in combustion control and technologies for eliminating injurious materials are difficult unless without knowing which combustion condition an incinerator is operating in, and which components are contained how much in particular combustion off-gas.
Combustion off-gas may typically include a variety of components such as nitrogen, oxygen, water, carbon dioxide, carbon monoxide, nitrogen oxides (NOx), sulfur oxides (SOx), hydrocarbons, organochloric compounds, and so on in a variety of concentrations ranging from several tens of percent to a ppt (10xe2x88x9212) level. These components each vary largely in concentration and composition depending on a combustion condition. Generally, the temperature in an incinerator or a refuse furnace, and the concentration of oxygen (O2), carbon monoxide (CO) or the like have been conventionally used as measures of combustion in a garbage incineration facility. These components have been used as the measures mainly because they are relatively easy to detect.
However, at present, there exists no detector which is capable of measuring in real time the concentrations of all components in combustion gas or combustion off-gas other than oxygen, carbon monoxide and so on. Thus, garbage incinerators and so on have widely employed a method of controlling the combustion so as to minimize the concentration of carbon monoxide (CO) which is easy to detect.
JP-A-5-99411, for example, discloses a system for controlling garbage combustion based on the concentration of carbon monoxide as mentioned above. The disclosed garbage incineration control system determines whether the amount of sprayed water supplied into an incinerator and the amount of air supplied to the incinerator are excessive or insufficient from the temperature of the incinerator and the concentration of carbon monoxide. This system comprises a control amount calculation unit for generating supply control signals for the amount of sprayed water and the amount of supplied air from the temperature and the concentration of carbon monoxide, and supply control means for adjusting the amount of sprayed water and the amount of air in response to the respective supply control signals. In this way, the system can suppress unburned fuel.
On the other hand, a method of monitoring hydrocarbons contained in combustion off-gas is disclosed in JP-A-8-200658. The techniques disclosed in JP-A-8-200658 involve introducing combustion off-gas into a mass spectrometer or the like for analysis, and measuring the amount of carbon dioxide, a particular type of hydrocarbon, or the like contained in the combustion off-gas to monitor the combustion condition.
In the respective prior art techniques cited above, attempts have been made to detect some components or an unburned fuel in combustion off-gas from an incinerator, an internal combustion engine or the like to monitor the combustion condition or suppress exhausted injurious materials. However, they do not show any means for totally detecting components of combustion off-gas in real time to determine the combustion condition.
The employment of the concentration of carbon monoxide as a measure for combustion control, which is shown in JP-A-5-99411 and is most widely spread at present, implies many problem as the monitoring of combustion condition, since all components of combustion off-gas are not detected.
For example, in garbage incineration, unburned components produced by burning garbage may be roughly classified into aliphatic hydrocarbons (saturated hydrocarbons, unsaturated hydrocarbons, and so on), aromatic hydrocarbons, chlorination thereof, and so on. Among these components, the aliphatic hydrocarbon is burned in an initial state of combustion and transformed into carbon dioxide and water. However, if oxygen is not sufficiently supplied during the combustion (incomplete combustion), hydrocarbons may be partially oxidized, or aromatic hydrocarbons may be produced due to thermal decomposition or the like. Since the produced aromatic hydrocarbons are chemically stable compounds as compared with the aliphatic hydrocarbons, there is a tendency that the aliphatic hydrocarbons are first oxidized and burned while the aromatic hydrocarbons remain unburned in the incinerator.
Therefore, if a varying amount of supplied air during garbage incineration results in a change in the combustion condition, the concentration of carbon monoxide also varies depending on an excessive or insufficient amount of supplied air. On the other hand, variations in the concentration of aliphatic hydrocarbons and aromatic hydrocarbons as unburned components do not have a direct correlation to the amount of supplied air. Stated another way, the concentration of carbon monoxide merely indicates overs and shorts in the amount of supplied air.
Also, while it is believed that the dioxin group is produced by a reaction of aromatic hydrocarbons with hydrogen chloride, the concentration of detected carbon monoxide does not of course include information directly related to the generation of dioxin group by the reaction of aromatic hydrocarbon with hydrogen chloride. It is further said that there is no correlation between the dioxin group and carbon monoxide when the concentration of carbon monoxide is equal to or lower than 50 ppm. Stated another way, the combustion condition monitoring based on the concentration of carbon monoxide cannot measure or estimate the production of the dioxin group.
JP-A-8-200658 describes techniques for identifying components such as butadiene, benzene, toluene and so on, other than carbon dioxide and oxygen, from the mass numbers of ions based on the mass spectrum of combustion off-gas, and measuring ion current for these identified components to monitor the combustion condition. However, a large number of organic compounds, which are many unburned components, exist in the combustion off-gas. During ionization in a mass spectrometer, a large number of ions produced from many components are dispersed over a wide mass range, and their signals overlap one another. It is therefore difficult to identify the amount of ions for a single component from the amount of ions having a single mass number. For example, even in a hydrocarbon group, an ion having the mass number 128 may be naphthalene (C10H8) or nonane (C9H20). Moreover, ions having the mass number 84 with the same composition of C6H12 can be any of a plurality of compounds such as cyclohexane, hexene and so on. On the other hand, since the electron ionization (EI) frequently used in mass spectrometers involves impact of high energy electrons for ionization, many fragment ions are produced over a wide mass range in addition to molecular ions. The thus produced fragment ions further exacerbates the overlapping of components, thus making it difficult to identify components only in reliance on the mass numbers of ions. As a result, monitoring a particular component of interest will cause a confusion due to uncertainty in identification of ions.
When plastic garbage and/or crude garbage are burned, a change in the absolute amounts of components of combustion off-gas and a change in composition will result depending on ingredients of the garbage. In this event, a change in the concentration of ions of a particular component is not caused by a change in the combustion condition but by a change in composition of combustion off-gas. Particularly, when the composition of fuel (garbage), the moisture content and so on largely vary as in a garbage incineration facility and an industrial waste incineration facility, the techniques disclosed in JP-A-8-200658 will experience difficulties in monitoring the combustion condition.
It is therefore an object of the present invention to provide a method and apparatus which are capable of monitoring a combustion condition from the mass spectrum of combustion gas or combustion off-gas without affected by a change in fuel, combustion conditions or the like.
To achieve the above object, the present invention provides in a first aspect a combustion condition monitoring method for monitoring a combustion condition by collecting combustion off-gas produced by combustion, ionizing the combustion off-gas, and detecting a signal strength associated with an ion for each mass-to-charge with a mass spectrometer. The method comprises the steps of calculating a degree of unsaturation for an ion detected by the mass spectrometer from the mass-to-charge ratio of the ion, and accumulating signal strength values associated with ions having the same degree of unsaturation for each degree of unsaturation.
In a second aspect, the present invention provides a combustion condition monitoring system which includes collecting means for collecting combustion off-gas produced by combustion, an ion source for ionizing the combustion off-gas from the collecting means, a mass spectrometer for mass analyzing ions generated by the ion source to measure ion currents, and data processing means for processing measured signals to provide a mass spectrum corresponding to the sample gas. The data processing means may include means for calculating a degree of unsaturation for each ion on the created mass spectrum from the mass-to-charge ratio of the ion, and means for accumulating ion current values indicated on the mass spectrum for each degree of unsaturation.
Specifically, the present invention analyzes combustion off-gas in accordance with the mass spectrometry, converts the results of the analysis to the degree of unsaturation for processing, and monitors hydrocarbons contained in the combustion off-gas to readily keep track of the combustion condition.